Bubble Stability Research Articles

Film Blowing of Linear and Long-Chain Branched Poly(ethylene terephthalate).

Film blowing of Poly(ethylene terephthalate) (PET) is challenging due its inherently low melt viscosity and poor melt strength. In this study, it is shown how the rheological properties of a commercial PET can be altered by reactive extrusion using either pyromellitic dianhydride (PMDA) or a multifunctional epoxy (Joncryl® ADR 4368) as chain extender, in order to improve the processing behavior during film blowing. The modified materials were characterized by shear and elongation rheometry and relevant processing characteristics, like melt pressure, bubble stability, and film thickness uniformity, were used to assess the influence of the type of modifier on processing and product performance. It is shown that PMDA is useful to increase the melt strength which leads to an improved bubble stability, while epoxy modified PET shows a reduced drawability that can cause problems at high take-up ratios. On the other hand, the epoxy modifier indicates a pronounced strain hardening during elongational deformation, and therefore leads to a better film thickness uniformity compared to the neat PET and the PET modified with PMDA. The differences with respect to processing performance are discussed and ascribed to the molecular structure of the materials.

Effect of MWCNT carboxyl functionalization on the shear rheological and electrical properties of HMS-PP/MWCNT foams

Different concentrations of multiwall carbon nanotubes (MWCNT) and carboxyl functionalized MWCNT (MWCNT-COOH) were added to a high melt strength polypropylene (HMS-PP) to produce foams with high dielectric constants, using azodicarbonamide (AZO) as blowing agent. The AZO foaming behavior and the crystallization, thermal properties, steady state and oscillatory shear rheological properties of the nanocomposites were analyzed by polarized light optical microscopy (PLOM), differential scanning calorimetry (DSC), thermogravimetry analyses (TGA) and parallel plate rheometry. The morphology, the dielectric and dynamic mechanical properties (DMTA) of the foams were also studied by scanning electron microscopy (SEM), impedance spectroscopy and bending method, respectively. A decrease in crystallite size and an increase in the HMS-PP overall crystallinity promoted by the presence of both types of MWCNTs was observed, as well as an increase in the crystallization temperatures. From these results and from the analyses of the rheological properties, it was possible to predict that the 5 wt.% MWCNT foam would have the lowest bubble growth rate, the 1.5 wt.% MWCNT-COOH the highest, while the 3 wt.% MWCNT-COOH composition would have the slowest bubble stability (and consequently the highest cellular density) and the 1.5 wt.% MWCNT-COOH the fastest. Also, it was possible to predict that only the 5 wt.% MWCNT-COOH foam would have a percolated and electrically conductive structure. All these predictions were confirmed by the resultant morphology and impedance spectroscopy results. The highest mechanical damping was displayed by the 3 wt.% MWCNT-COOH foam, while the lowest by the 5 wt.% MWCNT-COOH foam. Regarding the dielectric properties, the 1.5 wt% MWCNT-COOH foam was found to be the most suitable to be used as a capacitor material; this foam was also the less dense of all the samples.

Optimal subharmonic emission of stable bubble oscillations in a tube.

The subharmonic acoustic emission of a stable oscillating bubble inside a rigid tube is investigated by direct numerical simulation. The mechanisms of bubble-tube interaction on the acoustic wave emitted by the bubble are clarified. When the bubble is small compared to the tube diameter, a critical threshold for the pressure amplitude appears beyond the point which nonspherical effects become important and bubble breaks. For a finite tube diameter, the scattered wave by the bubble is shown to generate a plane wave where the intensity of the subharmonic component becomes maximum for an optimal distance between the bubble and the tube wall. This effect seems to be directly related to the appearance of local resonance phenomena and a bubble resonance shift where liquid's compressibility plays a major role.

Performance comparison of swirl-venturi bubble generator and conventional venturi bubble generator

The bubble generation performance and liquid volumetric mass transfer coefficient kLa of the venturi microbubble generator and the swirl-venturi microbubble generator were evaluated through the high-speed photography and measurement of dissolved oxygen concentration. The results show that the swirl-venturi microbubble generator generates more bubbles with smaller Sauter mean diameter than the venturi microbubble generator. And the maximum stable bubble size of the swirl-venturi microbubble generator is also smaller than that of the venturi microbubble generator. Moreover, the maximum stable bubble size of the swirl-venturi microbubble generator is not sensitive to the increase of superficial gas velocity, which indicates that the swirl-venturi microbubble generator has a higher probability of breaking bubbles. Eventually, the liquid volumetric mass transfer coefficient kLa of swirl-venturi microbubble generator is measured and higher than that of the venturi microbubble generator at high superficial gas velocity. Based on all above, it can be concluded that the swirl-venturi microbubble generator is a more effective way to generate microbubbles.

Bubbles determine the amount of alcohol in Mezcal

Mezcal is a traditional Mexican spirit, obtained from the distillation of fermented agave juices. Its preparation has been conducted for centuries in an artisanal manner. The method used to determine the correct alcohol content is of particular interest: a stream of the liquor is poured into a small vessel to induce surface bubbles. These bubbles, known as pearls by the Mezcal artisans, remain stable for tenths of seconds only if the alcohol content is close to 50%. For higher or lower alcohol content, the bubbles burst rapidly. The long bubble lifetime is the result of surfactant-induced surface tension changes. However, the precise mechanism and its relation to alcohol content remain unexplained. In this investigation, the extended lifetime of pearls was studied both experimentally and numerically. It was found that changes in surface tension, density, viscosity (resulting from mixing ethanol and water), and the presence of surfactants are all relevant to extend the bubble lifetime. The dimensionless bubble lifetime was found to reach its maximum value when the Bond number was close to unity, corresponding to 2 mm Mezcal bubbles. These findings show that the traditional empirical method does work. Beyond this, the understanding of the process provides physical insight to many other natural and industrial problems for which the stability of surface bubbles is of importance, such as bio-foams, froth floatation, and volcanic flows.

How does the presence of stevia glycosides impact surface bubbles stability?

The addition of sweeteners in fizzy beverages not only affects the sugar content but also the bubbles stability. In this article, we propose a model experiment, in which the lifetime of hundreds of single bubbles is measured, to assess the stability of bubbles in solutions containing either sucrose or sweeteners. We show that the bubbles are indeed more stable in presence of sweeteners, which are surface active molecules and adsorb at the interface. Additionally, we test an antifoam at different concentrations and show that our experiment allows to identify the best concentration to reproduce the stability obtained in sucrose when we replace this latter by a sweetener.

Spatially-directed cell migration in acoustically-responsive scaffolds through the controlled delivery of basic fibroblast growth factor

Hydrogels are commonly used in regenerative medicine for the delivery of growth factors (GFs). The spatial and temporal presentations of GFs are critical for directing regenerative processes, yet conventional hydrogels do not enable such control. We have developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), where release of a GF is non-invasively and spatiotemporally-controlled using focused ultrasound. The ARS consists of a fibrin matrix doped with a GF-loaded, phase-shift emulsion. The GF is released when the ARS is exposed to suprathreshold ultrasound via a mechanism termed acoustic droplet vaporization. In this study, we investigate how different spatial patterns of suprathreshold ultrasound can impact the biological response upon in vivo implantation of an ARS containing basic fibroblast growth factor (bFGF). ARSs were fabricated with either perfluorohexane (bFGF-C6-ARS) or perflurooctane (bFGF-C8-ARS) within the phase-shift emulsion. Ultrasound generated stable bubbles in bFGF-C6-ARS, which inhibited matrix compaction, whereas transiently stable bubbles were generated in bFGF-C8-ARS, which decreased in height by 44% within one day of implantation. The rate of bFGF release and distance of host cell migration were up to 6.8-fold and 8.1-fold greater, respectively, in bFGF-C8-ARS versus bFGF-C6-ARS. Ultrasound increased the formation of macropores within the fibrin matrix of bFGF-C8-ARS by 2.7-fold. These results demonstrate that spatially patterning suprathreshold ultrasound within bFGF-C8-ARS can be used to elicit a spatially-directed response from the host. Overall, these findings can be used in developing strategies to spatially pattern regenerative processes. Statement of SignificanceHydrogels are commonly used in regenerative medicine for the delivery of growth factors (GFs). The spatial and temporal presentations of GFs are critical for directing regenerative processes, yet conventional hydrogels do not enable such control. We have developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), where GF release is non-invasively and spatiotemporally-controlled using focused ultrasound. The ARS consists of a fibrin matrix doped with a phase-shift emulsion loaded with GF, which is released when the ARS is exposed to ultrasound. In this in vivo study, we demonstrate that spatially patterning ultrasound within an ARS containing basic fibroblast growth factor (bFGF) can elicit a spatially-directed response from the host. Overall, these findings can be used in developing strategies to spatially pattern regenerative processes.

Comparison of the stabilities of a soap bubble and the ball lightning in the form of a bubble of light

A bubble of circulating light is an optical model of natural ball lightning. We show that the bubble of light consisting of a thin spherical shell of highly compressed air, in which intense white light circulates in all possible directions, can be stable at various proportions between the energies of light and compressed air. Correctness of the model is based on the numerous evidence that its behavior in the terrestrial atmosphere coincides completely with the intriguing and mysterious behavior of ball lightnings. We believe that natural ball lightnings can be considered as an experimental confirmation of an existence of bubbles of light. Because of this an analysis of stability of the model is a task with known in advance answer. At the same time, the scientific community has great doubts about the existence of bubbles of light in nature, because no one has studied such objects either theoretically or experimentally. The article is intended mainly for specialists in optics. Since the issues of stability of circulating light are not included in the range of basic issues of modern optics, a discussion of these issues is presented in detail. For this purpose, an analysis of stability of an ordinary soap bubble, the stability of which is not in doubt, is preliminary presented. Then the same concepts, methods, and notation are used to prove the stability of the bubble of light.

Micro–nanobubble technology and water-related application

AbstractCurrently, there is a growing demand for water treatment technologies considering global environmental challenges such as degradation and depletion of water resources. Micro- and nanobubble (MNB) technology and its application for wastewater treatment has emerged as a problem-solving alternative for such challenges. This paper reviews the important studies on water treatment in the areas of MNBs and discusses their fundamental properties, such as bubble stability (as tiny entities in water solutions), generation methods, and various chemical and physical features. The paper further overviews the current status of MNB application in water treatment processes such as flotation, aeration, and disinfection and its uses in various sectors, including agriculture, aquaculture, medical, and industry. Based on this review, studies regarding MNBs' basic properties, generation, and application are identified and recognized for future research. This study concludes that despite the promising role of MNBs in water-related application, the current status of research has not reached its true potential. Specifically, there is a need to enhance MNB application at a broader scale.

Experimental description of bubble dynamics and heat transfer processes occurring on the pool boiling of water on biphilic surfaces

This study concerns the detailed description of the fluid dynamics and heat transfer mechanisms occurring during single bubble nucleation on biphilic surfaces (superhydrophilic/hydrophilic surfaces surrounding hydrophobic/superhydrophobic isles). A high-speed video camera is synchronized with a high-speed thermographic camera to relate the temporal evolution of bubble dynamics, from generation to detachment. The results allowed identifying several stages of bubble growth in all the biphilic surfaces tested, regardless of the size of the superhydrophilic regions. Bubble dynamics is affected by the size of the superhydrophobic regions. Since bubbles are constrained to the superhydrophobic region boundaries, which alter their base diameter, smaller superhydrophobic regions tend to promote a regular and stable bubble generation, due to the action of stronger surface tension forces acting on the boundary region. As the base diameter increases, the surface tension effects, which only act at the boundary with the hydrophilic region, are lessened and only affect the bubble at larger volumes. Consequently, smaller superhydrophobic regions are associated to higher evaporation mass transfer rates. Temperature gradients are larger at the hydrophilic/superhydrophobic boundaries and despite being small amplitude differences, they clearly promote induced convection of the cold liquid between superhydrophobic regions, as observed in the high-speed thermographic images.

Fast transformation of 2D nanofiber membranes into pre-molded 3D scaffolds with biomimetic and oriented porous structure for biomedical applications.

The ability to transform two-dimensional (2D) structures into three-dimensional (3D) structures leads to a variety of applications in fields such as soft electronics, soft robotics, and other biomedical-related fields. Previous reports have focused on using electrospun nanofibers due to their ability to mimic the extracellular matrix. These studies often lead to poor results due to the dense structures and small poor sizes of 2D nanofiber membranes. Using a unique method of combining innovative gas-foaming and molding technologies, we report the rapid transformation of 2D nanofiber membranes into predesigned 3D scaffolds with biomimetic and oriented porous structure. By adding a surfactant (pluronic F-127) to poly(ε-caprolactone) (PCL) nanofibers, the rate of expansion is dramatically enhanced due to the increase in hydrophilicity and subsequent gas bubble stability. Using this novel method together with molding, 3D objects with cylindrical, hollow cylindrical, cuboid, spherical, and irregular shapes are created. Interestingly, these 3D shapes exhibit anisotropy and consistent pore sizes throughout entire object. Through further treatment with gelatin, the scaffolds become superelastic and shape-recoverable. Additionally, gelatin-coated, cube-shaped scaffolds were further functionalized with polypyrrole coatings and exhibited dynamic electrical conductivity during cyclic compression. Cuboid-shaped scaffolds have been demonstrated to be effective for compressible hemorrhage in a porcine liver injury model. In addition, human neural progenitor cells can be uniformly distributed and differentiated into neurons throughout the cylinder-shaped nanofiber scaffolds, forming ordered 3D neural tissue constructs. Taken together, the approach presented in this study is very promising in the production of pre-molded 3D nanofiber scaffolds for many biomedical applications.

Effects of acoustic fields on the dynamics of micron-sized particles in a fluidized bed

The effect of external acoustic fields on hydrodynamic behavior of cohesive micron-sized particles in a pseudo-2D fluidized bed is studied using CFD-DEM simulations. The forces acting on a particle consist of the contact force and cohesive force between particles, the drag force by the surrounding fluid and the sound force by an acoustic field. A wide range of sound pressure levels and sound frequencies are investigated in terms of their effects on the pressure drop, bed expansion, distribution of particle velocity and concentration, as well as the sizes of bubbles and agglomerates. Comparing to the system without acoustic force, the application of acoustic field is found to induce the breakup of the formed agglomerates, and ultimately lead to a stable bubbling fluidization regime. A useful range of frequencies between 80 Hz to 100 Hz is found with a pressure level of 120 dB, resulting in most remarkable improvement of the process performance.

Acoustic noise spectra under hydrothermal conditions.

Acoustic noise spectra were studied for the first time in overheated water using sonohydrothermal reactor operating at 20kHz ultrasound in the temperature range from 25 to 200°C at the autogenic pressure of 1-14bar. The obtained results highlighted a dominating role of stable cavitation during ultrasonic treatment of hot water. Heating of sonicated water results in the formation of large number of nonlinearly oscillating bubbles synchronous with the driving frequency. At 200°C, the acoustic spectra also display strong subharmonic and multiple ultraharmonic bands. Moreover, cavitation bubbles formed at 200°C exhibit chaotic and random motions. It has been shown that the addition of TiO2 nanoparticles to hydrothermal water heated at 200°C allows to eliminate subharmonic/ultraharmonic bands and stochastic oscillations as well. This effect was assigned to Pickering-like bubble stabilization due to the particle accumulation at the bubble surface.

Flotation-based dye removal system: Sweet potato protein fabricated from agro-industrial waste as a collector and frother

Hazardous synthetic dyes have emerged as an imperative issue of water contamination. This investigation focused on the development of a green flotation-based dye removal (FBDR) system to remove trace crystal violet (CV). A natural protein derived from sweet potato waste which was a readily available and low-cost agro-industrial residual was utilized as the collector and frother, due to its biodegradability and nontoxicity. In the first-stage of FBDR, an inclined foam channel with hydrophobic surface was employed to continuously recover sweet potato protein (SPP). Its recovery percentage and volume ratio reached 84.7 ± 4.2% and 7.2 ± 0.4, respectively. Enriched SPP (10.4 ± 0.5 g/L) was then collected by isoelectric precipitation and freeze drying. Encouragingly, SPP induced the desirable foaming and stable bubbles of the solution. Analytical tests, including SEM, FTIR, UV–vis spectra, and fluorescence measurements, confirmed that the SPP-CV complex was formed mainly through the hydrophobic force. In the second-stage, an excellent CV removal was generated with the collection of SPP. The removal efficiency and enrichment ratio of CV were 95.8 ± 4.5% and 9.2 ± 0.4, respectively. The kinetic study indicated that the removal rate markedly increased with increasing air flowrate. In conclusion, this investigation demonstrated the waste protein could be an effective, inexpensive and eco-friendly alternative for classical collector and/or frother, and provided new insights into the disposal of dye wastewaters.

In situ characterisation of size distribution and rise velocity of microbubbles by high-speed photography

Using microbubbles has gained significant interest in many domestic and industrial applications due to bubble stability in solution and increased mass transfer area. The characterisation of microbubble populations is therefore important and aids in the understanding of their behaviour. Microbubble characterisation remains challenging, particularly at high bubble densities. We have developed an in situ and automated method, based on image analysis, to determine bubble size distributions and bubble rise velocity at bubble densities of up to approximately 7 bubbles mm−2. The method uses image analysis of a side-stream viewing slit and was tested using air bubbles in water at diameters between 20 and 150 µm under a range of different conditions. The developed system enables fast, simple and accurate size determination for microbubbles, including continuous sampling and observation.

Shape stability of a gas bubble in a soft solid.

Predicting the onset of non-spherical oscillations of bubbles in soft matter is a fundamental cavitation problem with implications to sonoprocessing, polymeric materials synthesis, and biomedical ultrasound applications. The shape stability of a bubble in a Kelvin-Voigt viscoelastic medium with nonlinear elasticity, the simplest constitutive model for soft solids, is analytically investigated and compared to experiments. Using perturbation methods, we develop a model reducing the equations of motion to two sets of evolution equations: a Rayleigh-Plesset-type equation for the mean (volume-equivalent) bubble radius and an equation for the non-spherical mode amplitudes. Parametric instability is predicted by examining the natural frequency and the Mathieu equation for the non-spherical modes, which are obtained from our model. Our theoretical results show good agreement with published experiments of the shape oscillations of a bubble in a gelatin gel. We further examine the impact of viscoelasticity on the time evolution of non-spherical mode amplitudes. In particular, we find that viscosity increases the damping rate, thus suppressing the shape instability, while shear modulus increases the natural frequency, which changes the unstable mode. We also explain the contributions of rotational and irrotational fields to the viscoelastic stresses in the surroundings and at the bubble surface, as these contributions affect the damping rate and the unstable mode. Our analysis on the role of viscoelasticity is potentially useful to measure viscoelastic properties of soft materials by experimentally observing the shape oscillations of a bubble.

Low frequency ultrasound as a potentially viable foaming option for pathological veins

An investigation was carried out on bubble size distributions of sclerosing foams exploiting different methods of production. A lot of effort has been dedicated by researchers over the past two decades in producing foams with better safety profiles to reduce the risk of ischemic events. There are manual and automated techniques, and the state-of-art suggests that an optimal foam has not yet been produced. Herein, a comparison is performed on sclerosing foams produced using the marketed Easyfoam® kit or low frequency ultrasound. Each technique provides a final foam with specific physical characteristics, such as bubble size distribution and stability. This is the first characterization about the bubble size distributions on foams using the manual method clinically available. In this paper, foam bubble sizes were investigated using 4% polidocanol solutions, and different power levels in the case of the ultrasound technique. Specific comparisons were made of the size distributions, the changing bubble diameter over time (up to 2 min) and the liquid fraction. In the formal theoretical analysis, a simple model was proposed with the aim of estimating the number of bubbles that would be required to provoke occlusion events after foam injections. As for the experimental novelty, the foam characterization evidenced that ultrasound generated considerably smaller bubble diameters, producing very wet foams with narrower distributions. Moreover, sonicated foams were less prone to ageing, generating more stable bubbles over time. Such a foam may be applicable to the treatment of small pathological veins (e.g., cosmetic veins or reticular veins), which are currently most often treated with liquid injections.

Effect of viscosity and shear regime on stability of the air-void system in self-consolidating concrete using Taguchi method

Achieving suitable air-void characteristics in self-consolidating concrete (SCC) can be a difficult task due to the various influencing factors, including rheology and shear history. The effect of viscosity and shear regime on air-void system (AVS) of different SCC mixtures was evaluated using Taguchi method. The investigated SCC mixtures were proportioned with various high-range water-reducer (HRWR) and viscosity modifying admixture (VMA) combinations. Three different shear regimes, corresponding to static, semi-static, and dynamic were evaluated. These regimes simulate the concrete at rest, transportation, and casting processes, respectively. The characteristics of AVS were investigated immediately after mixing and up to 60 min after the first contact between water and binder using the ASTM C231 and air-void analyzer (AVA) test methods. Test results showed that the viscosity and shear rate regime are key factors affecting the AVS in SCC. High viscosity SCC mixtures exhibited better stability of air bubbles and spacing factor, regardless of the shear regime. On the other hand, the maximum size of aggregate showed the lowest contribution. Based on the statistical analysis, the optimum levels of control factors to minimize the variation in air volume (up to 60 min) and spacing factor are quantified. This consists, by order of importance, of the static shear regime and high viscosity of the mixture. Under these conditions, a high effective air fraction can be secured. Indeed, greater effective fraction of air bubbles is observed in high viscosity SCC compared to that of low viscosity mixtures.

Insight into selection of appropriate formulation for colloidal gas aphron (CGA)-based drilling fluids

Application of light-weight drilling fluids is essential to develop depleted hydrocarbon reservoirs. Recently, colloidal gas aphron (CGA)-based fluids have been introduced for such applications due to their ability in controlling fluid losses. In this work, a comprehensive experimental study was performed to choose the best formulation for CGA fluids by implementing static stability tests, rheological behavior measurements, and bubble size analyses of CGAs. Xanthan gum polymer and sodium dodecyl benzene sulfonate (SDBS), an anionic surfactant, and cetyl trimethyl ammonium bromide (CTAB), a cationic surfactant, were utilized to prepare CGAs. For the range of experiments conducted, the performance of CGA fluids prepared with SDBS was improved by increasing the polymer and surfactant concentrations. However, for CTAB, it was improved by an increase in the polymer concentration and a decrease in the surfactant concentration. The formation of white, long hair-like precipitates observed at high levels of CTAB caused CGA fluid to become rapidly unstable. Also, it was observed that the size of CGAs was significantly influenced by the polymer and surfactant concentrations. The most stable bubbles were formed at 6.86 g/L of polymer concentration. The results of this study provide insights into appropriate formulation for CGA-based fluids which could be potentially applicable in drilling operations.

Foam-free monodisperse lipid-coated ultrasound contrast agent synthesis by flow-focusing through multi-gas-component microbubble stabilization

Monodisperse lipid-coated microbubbles are a promising avenue to unlock the full potential of ultrasound contrast agents for medical diagnosis and therapy. However, their formation by microfluidic flow-focusing is non-trivial. The lipid monolayer shell around the freshly formed bubbles is initially loosely packed, resulting in gas exchange between bubbles through Ostwald ripening, eventually leading to the formation of large, potentially thrombogenic, foam bubbles. Here, we show that by formulating a gas mixture of a low- and a high-aqueous solubility gas, a microbubble suspension can be formed that is not only monodisperse and highly stable, but it can also be synthesized without foam bubble formation at clinically relevant concentrations. The optimal gas volume fraction and resulting gas composition of the stable bubbles are modeled and were found to be in excellent agreement with the experimental data. This physics approach to an interfacial chemistry problem therefore opens a route to bedside production of stable, safe, and readily injectable monodisperse bubbles for medical applications.